The iron cross stretch is a lower back stretch that warms-up and lengthens the muscles in the lower back. The stretch can be held as a static stretch after a workout, or it can be done at a rapid pace as a dynamic warm-up.
This stretch is done by laying on your back, raising a leg straight into the air and placing lowering it back to the ground across your body. You should concentrate on keeping both of your shoulders on the surface of the floor and getting your leg as far across your body as you can.
You will feel the stretch in your lower back. If your leg is straight you may also feel a stretch in your hamstring. If you have stiff joints in your back and hips this may help mobilize those joints as well.